1) Field of the Invention
The subject invention relates to an assembly for measuring movement of a shaft and for measuring a magnetic flux transmitted through the shaft as a result of a torque applied thereto.
2) Description of Related Art
In systems having rotating drive shafts it is sometimes necessary to know the torque, position, and speed of these shafts in order to control the same or other devices associated with the rotatable shafts. Accordingly, it is desirable to sense and measure the torque applied to these items and their positions in an accurate, reliable and inexpensive manner. Sensors to measure the torque imposed on rotating shafts, such as but not limited to shafts in vehicles, are used in many applications. For example, it might be desirable to measure the torque on rotating shafts in a vehicle's transmission, or in a vehicle's engine (e.g., the crankshaft), or in a vehicle's automatic braking system (ABS) for a variety of purposes known in the art.
One application of this type of torque measurement is in electric power steering systems wherein an electric motor is driven in response to the operation and/or manipulation of a vehicle steering wheel. The system then interprets the amount of torque or rotation applied to the steering wheel and its attached shaft in order to translate the information into an appropriate command for an operating means of the steerable wheels of the vehicle.
Prior methods for obtaining torque measurement in such systems were accomplished through the use of contact-type sensors directly attached to the shaft being rotated. For example, one such type of sensor is a “strain gauge” type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and the applied torque is measured by detecting a change in resistance, which is caused by applied strain and is measured by a bridge circuit or other well-known means.
Another type of sensor used is a non-contact torque sensor. These non-contact torque sensors have a magnetostrictive (MR) material, or coating material, disposed on rotating shafts and sensors are positioned to detect the presence of an external flux which is the result of a torque being applied to the magnetostrictive material. Such magnetostrictive materials require an inherent magnetic field within the material which is typically produced or provided by pre-stressing. Forces are applied (e.g., compressive or tensile forces) to pre-stress the coating prior to magnetization of the pre-stressed coating in order to provide the desired magnetic field. Alternatively, an external magnet or magnets are provided to produce the same or a similar result to the magnetostrictive material. To this end, magnetostrictive torque sensors have been provided wherein a sensor is positioned in a surrounding relationship with a rotating shaft, with an air gap being established between the sensor and shaft to allow the shaft to rotate without rubbing against the sensor.
However, these various related art assemblies attempt to obtain the circumferential component by providing the coating material having the proper magnetostrictive properties and having a capability of supporting a permanent magnetic moment, i.e., a magnetic coercivity. With the latter, the material could be permanently oriented magnetically via the temporary application of an external magnetic field. Finding a coating material that has both proper magnetostrictive properties and magnetic coercivity properties has proved elusive.
Accordingly, it would be advantageous to provide an assembly that did not require the coating material to have the proper magnetostrictive properties and a capability of supporting a permanent magnetic moment. It would also be advantageous to provide an assembly that could be formed of less expensive materials than those having the properties set forth above.